A need exists for a reliable low cost device for warning a vehicle driver of loss of air pressure in any of the vehicle tires. Such a warning system serves not only the purpose of safe vehicle operation but also it reduces damage to vehicle tires which results from running at low pressure. Many devices have been proposed but they have the disadvantage of being unreliable, too costly or too complicated.
In the prior art, it is known to provide a device on the valve stem of a tire for the purpose of warning the driver by an explosive report that the tire pressure is below a predetermined value. In general, such devices comprise a housing which is adapted to be screwed onto the filler valve stem of a tire and which contains a pressure sensing valve. A discharge passage in the housing is closed by an inflatable balloon and a closure valve is disposed in the discharge passage. So long as the tire pressure is above a predetermined value, the sensing valve remains closed. When the tire pressure drops below a predetermined value, the sensing valve is opened and the pressurized air from the tire is admitted to the balloon and it is inflated until it explodes. Upon explosion of the balloon a rush of air through the discharge passage causes the closure valve to be closed to prevent further loss of tire pressure. Devices of this type are described in the Su U.S. Pat. No. 4,024,829 granted May 24, 1977 and the Yu U.S. Pat. No. 4,155,325 granted May 22, 1979.
An improved warning device of the type described above is set forth in our copending patent application Ser. No. 478,321 filed Mar. 24, 1983 now U.S. Pat. No. 4,601,254 and entitled "TIRE PRESSURE WARNING DEVICE". The disclosure of that patent application is hereby incorporated herein by reference.
In the prior art, it is also known to utilize a device on the tire for transmitting the signal wave to a monitor on the vehicle when the tire pressure falls below a predetermined value.
A general objective of this invention is to provide an improved tire pressure warning system which overcomes certain disadvantages of the prior art.